WEDNESDAY, Oct. 24 (HealthDay News) -- It is said that timing is everything. When it comes to taking hormone replacement therapy, that may indeed be true.

A new study has found that taking hormone replacement therapy within five years of menopause may reduce the risk of developing Alzheimer's disease by 30 percent compared to women who have not used hormone therapy.

The researchers set out to better understand why some previous studies had shown hormones were effective in reducing Alzheimer's risk while others had not.

The Women's Health Initiative Memory Study, a large, multicenter trial, was stopped prematurely in 2002 due to concerns that the hormones might be raising the risks of cardiovascular disease, cancer and dementia.

"The Women's Health study put a cloud over doing further research," said Peter Zandi, author of the latest study and an associate professor at Johns Hopkins University Bloomberg School of Public Health, in Baltimore. "But there was still some mystery: Why were those results so different?"

The new study, performed by a different team of researchers, expanded the previous research by including up to seven years of additional follow-up and more detailed data on the duration, type and timing of hormone therapy.

The new research was published online Oct. 24 in the journal Neurology.

"We found there is likely a critical window after menopause when hormone therapy may be beneficial," Zandi said. "But if started later after menopause, hormones may actually increase the risk of developing Alzheimer's."

Why might that timing be important?

"We don't really know, but we have some ideas," Zandi said. "Around menopause there's a sudden decline in endogenous estrogens, and that may have some provocative effect. Alzheimer's is a chronic disease that most likely has roots in middle age, or earlier." Endogenous estrogens are those that are made naturally by the body.

The researchers followed nearly 1,800 women from Cache County, Utah, aged 65 and older, for 11 years. The participants provided a history of their hormone therapy use and the approximate date when menopause began.

In all, more than 1,100 participants had used hormone therapy, either consisting of estrogen alone or in combination with progestin. During the study, 7.9 percent of the women who had taken hormones developed Alzheimer's disease compared with 13.4 percent of the women who had not.

Although those who had begun hormone therapy within five years of menopause had a 30 percent lower risk of Alzheimer's than those who had not used hormones, a higher risk of dementia was observed among women who had started a combined therapy of estrogen and progestin when they were 65 or older.

The U.S. Preventive Services Task Force, an independent group of national experts in prevention and evidence-based medicine, issued a final statement Oct. 23 recommending against the use of hormone therapy to prevent chronic conditions such as heart disease or dementia in women who had experienced menopause.

Given that recommendation, and the wide range of research about hormone replacement therapy, what should women do?

Dr. Victor Henderson, professor of neurology and health research and policy at Stanford University, said the research shouldn't spur women to take hormone therapy in an effort to cut their risk of getting Alzheimer's.

"I don't think the message is that women of any age should take the hormones to reduce dementia risk," he said.

Henderson, who wrote an accompanying editorial in same journal issue, urged caution: "More research is needed before we can make new recommendations for women about their use of hormone replacement therapy," he said.